The invention relates to a switching device for energizing a lamp with a high-frequency lamp current at a frequency f, which switching device is provided with
a first circuit part for generating a periodical, substantially square-wave voltage at a frequency f, which first circuit part is provided with output terminals,
a load branch comprising
a first branch, which interconnects the output terminals, and which comprises a first series arrangement of a first inductive element and a first capacitate element,
a second branch, which shunts the first capacitate element, and which comprises a series arrangement of a second inductive element and a second capacitate element, and
a third branch, which comprises lamp terminals, and which shunts the second capacitate element during operation of the lamp.
Such a switching device is disclosed in U.S. Pat. No. 5,426,350. A high-frequency current is to be taken to mean a current with a frequency above 10 kHz. In the known switching device, both capacitate elements and both inductive elements are dimensioned such that, during stationary lamp operation, a substantially sinusoidal current flows through the lamp at a frequency f. In spite of the fact that the known switching device does not include a transformer, this sinusoidal current has a comparatively high amplitude as compared to the amplitude of the substantially square-wave voltage. By virtue thereof, it is possible to use the known switching device to supply a comparatively high burning voltage to a discharge lamp.
A drawback of the known switching device is, however, that the efficacy of the lamp, in other words the ratio between the luminous flux of the lamp and the power consumed by the lamp, is comparatively low.
Therefore, it is an object of the invention to provide a switching device for energizing a lamp, which transfers power to the lamp in such a way that the efficacy of the lamp is comparatively high.
To achieve this, a switching device as described in the opening paragraph is characterized in accordance with the invention in that the first branch and the second branch are dimensioned such that the following relation is met
0.14 less than xcexa3A(n)/A(1) less than 0.42,
wherein A(1) denotes the amplitude of the first harmonic component of the high-frequency lamp current, and xcexa3 A(n) is the sum of the amplitudes of the higher harmonic components of the lamp current.
Instead of being substantially sinusoidal, the high-frequency lamp current generated during lamp operation by a switching device in accordance with the invention has a shape in between that of a sine and a square-wave. It is known, for example from WO 96/19095, that a substantially square-wave lamp current enables a very high efficacy to be achieved. A drawback of such a substantially square-wave lamp current is the comparatively large amount of RFI generated by the lamp. As the current generated by a switching device in accordance with the invention is not substantially square-wave shaped, but rather has a shape in between that of a sine and a square wave, the amount of RFI generated by the lamp is comparatively small, while the efficacy is substantially higher than in the case of a substantially sine-shaped lamp current. In addition, a switching device in accordance with the invention has a comparatively simple structure and hence is comparatively inexpensive.
Very good results are achieved with embodiments of a switching device in accordance with the invention, wherein the first branch and the second branch are dimensioned such that the following relation is met
0.21 less than xcexa3A(n)/A(1) less than 0.35.
It proved advantageous to provide the first circuit part with
input terminals which are to be connected to a DC voltage source,
a fourth branch comprising a series arrangement of two switching elements,
a control circuit coupled to control electrodes of the switching elements for
rendering the switching elements alternately conducting and non-conducting.
In this manner, the first circuit part is obtained in a comparatively simple and reliable manner.
To render such an embodiment of a switching device in accordance with the invention suitable for being energized with an AC voltage, the first circuit part may additionally be provided with
mains input terminals which are to be connected to a supply-voltage source supplying an AC voltage,
rectifier means, coupled to the mains supply terminals and to the input terminals, which rectifier means are used to rectify the AC voltage.
To preclude that comparatively much power is dissipated in the two switching elements, the frequency f is generally chosen to be higher than the resonance frequency of the load branch during operation of the lamp.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter.